Process for conversion of UF6 to UO2

ABSTRACT

A process for conversion of gaseous UF 6  to UO 2  powders by using a fluidized bed reaction apparatus comprising pyrohydrolizing gaseous UF 6  and steam to obtain UO 2  F 2  particles, hydrating and dehydrating the UO 2  F 2  particles to UO 2  F 2  anhydride and reducing the UO 2  F 2  anhydride to UO 2  powders. The obtained UO 2  powders are suitable for production of nuclear fuels in power plant owing to its good ceramic properties, low fluorine contents and free flowability.

BACKGROUND OF THE INVENTION

The present invention relates to a process for conversion of gaseous UF₆ to UO₂ powders which is suitable for production of nuclear fuels in power plant owing to its good ceramic properties, small fluorine contents and free flowability.

As a process for converting UF₆ into UO₂ powders for nuclear fuels in a power plant, gaseous UF₆ has been conventionally converted to be UO₂ powder in a industrial scale by two different process, that is, a wet process and a dry process. The wet process is defective in that many steps required make the operation complex and a large quantity of waste solution is produced.

On the other hand, the dry process has defects of having poor ceramic properties of UO₂ powders as a product and a large fluorine contents of the product, but it has advantages of having simple steps and also a small quantity of waste solution produced. Therefore, adoption of the dry process has been recently increased by overcoming the defects above mentioned. As the dry process, there are a process using a rotary kiln, a process using a fluidized bed reaction apparatus and a process using a flame combustion reaction apparatus. Of these processes, the process using a fluidized bed reaction apparatus produces UO powders as a product which has a free flowability, thus making handling of the UO₂ powders in following steps very much easier, as compared with that of the other processes.

In the process using a fluidized bed reaction apparatus which has the advantages mentioned, the ceramic properties of the UO₂ powders produced become poorer and also the fluorine contents thereof become larger, as compared with those of the other processes. The poor ceramic properties of the UO₂ powders is due to a formation of UO₂ F₂ in fine particles by gas phase reaction of gaseous UF₆ with steam as shown in the following equation (1) and a formation of UF₄ in converting of UO₂ F₂ to UO₂ with hydrogen gas as shown in the following equations (2) and (3). In the conventional dry processes, especially in the process using a fluidized bed reaction apparatus, the reaction is mostly composed of the following two stage reactions.

    UF.sub.6 +2H.sub.2 O→UO.sub.2 F.sub.2 +4HF          (1)

    UO.sub.2 F.sub.2 +H.sub.2 →UO.sub.2 +2HF            (2)

In this process, UF₄ is apt to be formed by a reverse reaction as shown in the equation (3). Namely, the UO powder may possibly be hydrofluorinated to UF powder.

    UO.sub.2 +4HF→UF.sub.4 +2H.sub.2 O                  (3)

UF₄ is a substance which is apt to sinter at a relatively low temperature (about 1000° C.) and begins to sinter at the operating temperature of the equation (2) to hinder a defluorinating reaction which is important for lowering fluorine contents of UO₂ powder as a product. Therefore, it was formerly required to add an excess of steam in the equation (2) to suppress hydrofluorination of the UO₂ powder. As a result, the fluidized bed operation became more complex and at the same time the excessively added steam increased a quantity of waste solution substantially. Further, as a long time was required for defluorinating UO₂ powder as a product, it was exposed to a high temperature for a long time. Consequently the ceramic properties of UO₂ powders were apt to be substantially reduced.

Further, another defect of the case in which the fluidized bed reaction apparatus is used relates to a stability of the operation of the fluidized bed which converts UF₆ into UO₂ F₂.

Namely, the UO₂ F₂ particles form the fluidized bed, but gaseous UF₆ blown into the fluidized bed reacts with steam as a fluidizing gas introduced through the bottom of the fluidized bed to form UO₂ F₂ particles which deposit on the surface of UO₂ F₂ particles already existing. As the result, the thus deposited UO₂ F₂ cause growth of the UO₂ F₂ particles. On the other hand, a part of the UO₂ F₂ particles are pulverized by abrasion owing to collison with each other. The mean particle size of the UO₂ F₂ particles is determined by these balances, but in the conventional fluidized bed reaction apparatus, the thus obtained UO₂ F₂ particles are apt to grow substantially. Therefore, it was required to supply new UO₂ F₂ particles into the fluidized bed to maintain a stable fluidized bed operation. Consequently, the process system became complicated and the operation also became troublesome.

BRIEF SUMMARY OF THE INVENTION

It is an object of the present invention to provide a process for conversion of gaseous UF₆ to UO₂ powders which is suitable for production of nuclear fuels in power plant owing to its good ceramic properties, low fluorine contents and free flowability.

According to the present invention, there is provided:

1. A process for conversion of UF₆ to UO₂ by using a fluidized bed reaction apparatus comprising

(a) a first step of pyrohydrolizing gaseous UF₆ and steam in said fluidized bed to obtain UO₂ F₂ particles,

(b) a second step of reacting said UO₂ F₂ particles with water to obtain UO₂ F₂ hydrate,

(c) a third step of dehydrating said UO₂ F₂ hydrate by heating to UO₂ F₂ anhydride,

(d) a fourth step of reducing said UO₂ F₂ anhydride with hydrogen gas or hydrogen gas and steam to convert to UO₂ powders.

2. A process for conversion of UF₆ to UO₂ by using a fluidized bed reaction apparatus comprising

(a) a first step of pyrohydrolizing gaseous UF₆ and steam in said fluidized bed to obtain UO₂ F₂ particles,

(b) a second step of hydrating said UO₂ F₂ particles with water to UO₂ F₂ hydrate,

(c) a third step of dehydrating said UO₂ F₂ hydrate by heating to UO₂ F₂ anhydride,

(d) a fourth step of calcining said UO₂ F₂ anhydride with steam to convert to UO₃ or a mixture of UO₃ and U₃ O₈ particles

(e) a fifth step of reducing said UO₃ or said mixture of UO₃ and U₃ O₈ with hydrogen gas or hydrogen gas and steam to convert to UO₂ powders.

Based on the above processes 1 and 2, following processes 3 and 4 are also within the scope of the present invention.

3. A process for conversion of UF₆ to UO₂ by using a fluidized bed reaction apparatus comprising

(a) a first step of pyrohydrolizing gaseous UF₆ and steam in said fluidized bed to obtain UO₂ F₂ particles,

(b) a second step of hydrating said UO₂ F₂ particles with water solution added with ammonia containing water, oxalic acid or hydrogen peroxide to UO₂ F₂ hydrate,

(c) a third step of dehydrating said UO₂ F₂ hydrate by heating to UO₂ F₂ anhydride,

(d) a fourth step of reducing said UO₂ F₂ anhydride with hydrogen gas or a mixture of hydrogen gas and steam to convert to UO₂ powders

4. A process for conversion of UF₆ to UO₂ by using a fluidized bed reaction apparatus comprising

(a) a first step of pyrohydrolizing gaseous UF₆ and steam in said fluidized bed to obtain UO₂ F₂ particles,

(b) a second step of hydrating said UO₂ F₂ particles with a solution added with ammonia containing water, oxalic acid or hydrogen peroxide to UO₂ F₂ hydrate,

(c) a third step of dehydrating said UO₂ F₂ hydrate by heating to UO₂ F₂ anhydride,

(d) a fourth step of reducing said UO₂ F₂ anhydride with steam to convert to UO₃ or mixture of UO₃ and U₃ O₈ partices,

(e) a fifth step of reducing said UO₃ or said mixture of UO₃ and U₃ O₈ with hydrogen gas or a mixture of hydrogen gas and steam to UO₂ powders.

Further, in the present invention, hydration of the UO₂ F₂ particles to UO₂ F₂ hydrate can be carried out with steam as a water source. Further more, in the present invention, supply of gaseous UF₆ and steam into the fluidized bed is carried out more effectively by using a twin fluid atomizer.

DETAILED DESCRIPTION OF THE INVENTION

In the present invention, the steps of converting gaseous UF₆ to UO₂ powders are as flollows:

    ______________________________________                                         the first step                                                                            UF.sub.6 + 2H.sub.2 O → UO.sub.2 F.sub.2                                                        (1)HF                                       the second step                                                                           UO.sub.2 F.sub.2 + nH.sub.2 O → UO.sub.2 F.sub.2.nH.sub.                2 O                     (2)                                         the third step                                                                            UO.sub.2 F.sub.2.nH.sub.2 O → UO.sub.2 F.sub.2 +                        nH.sub.2 O              (3)                                         the fourth step                                                                           UO.sub.2 F.sub.2 + H.sub.2 → UO.sub.2                                                           (4)HF                                       or                                                                             the first step                                                                            UF.sub.6 + 2H.sub.2 O → UO.sub.2 F.sub.2                                                        (1)HF                                       the second step                                                                           UO.sub.2 F.sub.2 + nH.sub.2 O → UO.sub.2 F.sub.2.nH.sub.                2 O                     (2)                                         the third step                                                                            UO.sub.2 F.sub.2.nH.sub.2 O → UO.sub.2 F.sub.2 +                        nH.sub.2 O              (3)                                         the fourth step                                                                           UO.sub.2 F.sub.2 + H.sub.2 O → UO.sub.3                                                         (4)HF                                                  UO.sub.2 F.sub.2 + H.sub.2 O → U.sub. 3 O.sub.8 + 2HF +                 1/6 O.sub.2                                                         the fifth step                                                                            UO.sub.3 + H.sub.2 → UO.sub.2 + H.sub.2 O                                                       (5)                                                    U.sub.3 O.sub.8 + 2H.sub.2 → 3UO.sub.2 + 2H                  ______________________________________                                                    O                                                              

Of these steps, the most important point involves the fact that in the second step, UO₂ F₂ particles formed in the fluidized bed of the first step are hydrated and then in the third step, the hydrated UO₂ F₂ particles are dehydrated by heating.

It is known that the UO₂ powders obtained by conversion of the dehydrated UO₂ F₂ particles have, a remarkably larger specific surface area and improved ceramic properties, but combination of the operation with the process of converting UF₆ to UO₂ using the fluidized bed can overcome the large defect of the conventional process that the UO₂ powders formed are unsuitable for fabrication of nuclear fuels in power plant owing to its low ceramic properties and slow defluorinating velocity can utilize also effectively the large advantage of the conventional process in which the UO₂ powders formed have an extremely good flowability.

Further, in the steps of converting UO₂ F₂ particles to UO₂ powders of the present invention, the UO₂ F₂ particles are calcined with only steam to UO₃ /U₃ O₈ to suppress a formation of HF₄ which hinders a defluorinating reaction of UO₂ F₂ and then the UO₃ /U₃ O₈ are reduced with hydrogen gas to UO₂ powders. Combination of these operations with the above stated conventional process can improve the defect thereof moreover.

The easily defluorinated UO₂ powders obtained by the present invention posses a fundamental condition suitable for fabrication of nuclear fuels in power plant owing to its good ceramic properties and at the same time have an extremely good flowability which makes handling thereof in following steps very easy and can omit a granulating operation which is usually carried out in fabrication of pellet for nuclear fuels.

Further, when a solution added with such reagent as ammonia, oxalic acid or hydrogen peroxide is used instead of water in hydration of the UO₂ F₂ particles, these reagents form uranate, uranyl salt. Therefore, the added quantity of these reagents can control the ceramic properties of UO₂ powders formed. Further, when steam is used as a water source instead of water, homogeneous hydration can be easily obtained due to its good dispersion and at the same time choice of apparatus can be more free. In the fluidized bed of the first step, it is effective to use a twin fluid atomizer for controlling the particle size of the formed UO₂ F₂ particles. This is due to the phenomenon that when the twin fluid atomizer is used, the gaseous UF atomized from central nozzle of the atomizer is pyrohydrolized with steam atomized from periphery of the nozzle to form fine particles of UO₂ F₂ which become nucleus of following granulation and average particle size of UO₂ F₂ particles which form the fluidized bed is decreased. Further, when the twin fluid atomizer is used, the formed UO₂ F₂ particles become highly reactive owing to its fine granule form to promote proceeding of following reactions.

The drawing is a schematic flow diagram of the process in accordance with the present invention. In the drawing, UF₆ is vaporized in a vaporizing chamber 1, and the gaseous UF₆ is atomized. At the same time, steam as a fluidizing gas is introduced into the bottom of the apparatus 2 through pipe 10. A part of gaseous UF₆ reacts immediately with steam as an atomizing gas near the atomizer to form UO₂ F₂ particle. A part of the newly fomred UO₂ F₂ particles are deposited on the surface of UO₂ F₂ as seed material, and the UO₂ F₂ particles are growing. Further, a part of these UO₂ F₂ particles is pulverized by collison with each other to be fine particles. The UO₂ F₂ partices are controlled in particle size by these phenomena and form the fluidized bed.

The operating temperature of the first fluidized bed 2 is less than 400° C., preferably in the range of 200°-300° C. in consideration of ceramic properties of the particles and control of the particle size. HF produced in the first fluidized bed 2 is recovered as HF solution by HF condenser 2a to be stored in HF receiver 2b. The newly formed UO₂ F₂ particles are discharged out of the first fluidized bed 2 through the overflow pipe which is located at the upper part of the fluidized bed 2 and are sent to the second reactor 3.

In the reactor 3, the UO₂ F₂ particles are hydrated with water introduced into the reactor 3 through pipe 11. The operating temperature is less than 100° C., preferably in the range of 10° C.-50° C. in consideration of hydration velocity.

The UO₂ F₂ hydrate is passed to chamber 4 from which it is sent to a third fluidized bed 5 where it is dehydrated by heating. Air as a fluidizing gas is introduced into the bottom of the apparatus 5 through pipe 13. The operating temperature is less than 200° C., preferably in the range of 120° C.-150° C. The formed UO₂ F₂ anhydride is discharged out of the fluidized bed 5 through the overflow pipe which is located at the upper part of the fluidized bed 5 and ssent to a fourth fluidized bed 6 where it is calcined with steam as reaction fluidizing gas introduced to the bottom of the apparatus 6 through pipe 14 to convert to UO₃ or U₃ O₈ particles. The operating temperature is less than 700° C., preferably in the range of 450° C.-600° C. at which UO₂ powders are formed. Further, when the operating temperature is in the range of 500° C.-600° C., a mixture of UO₃ and U₃ O₈ is formed. The UO₃ or U₃ O₈ particles are discharged out of the apparatus 6 through the overflow pipe and sent to a fifth fluidized bed 7 where they are reduced with a mixture of steam and hydrogen gas introduced to the bottom of the apparatus 7 through pipe 15 to convert UO₂ powders. The UO₂ powder is received as a product by a container 8. This operating temperature is less than 700° C., preferably in the range of 500° C.-600° C. in consideration of ceramic properties of the UO₂ powder.

The waste gas from the reaction apparatus 2, 5, 6, 7 is led to a waste gas treatment apparatus through a pipe 16.

The features of the present invention are as follows:

(1) The UO₂ powder formed has good ceramic properties, low fluorine contents and good flowability. Therefore, it is suitable for nuclear fuels in power plant. Such UO₂ powder as that of the present invention can not be obtained by conventional process.

(2) The good flowability of the UO₂ powder can make its handling in following steps very easy and can omit granulating operation before pelletizing which is generally carried out for fabricating nuclear fuels.

The following examples are illustractive of the present invention. However, it is understood that these examples are merely examplary and do not limit the scope the present invention.

Example

Three pilot runs were made using the processes by the present invention.

In case 1 corresponding to process 1, UO₂ F₂ particles formed by the first fluidized bed are hydrated, dehydrated and directly reduced to UO₂ powders.

In case 2 corresponding to process 2, UO₂ F₂ particles formed by the first fluidized bed are hydrated, dehydrated, calcined with steam to convert to UO₃ or U₃ O₈ and the UO₃ or U₃ O₈ are reduced to UO₂ powders.

In case 3 corresponding to process 3, UO₂ F₂ particles formed by the first fluidized bed are hydrated with ammonia containing water, dehydrated and reduced to UO₂ powders.

As a comparative example, one pilot run was made using the conventional dry process. In the comparative case, UO₂ F₂ particles formed by the first fluidized bed were directly reduced to UO₂ powders.

The fluidized beds used in these runs are 83 mm in diameter. The operating conditions are shown in Table 1. The characteristics of the UO₂ powders obtained in these runs are shown in Table 2.

Table 2 shows that the UO₂ powders of the process by the present invention are smaller in bulk density and in mean particle size, larger in specific surface area and lower in residual fluorine contents than those of the conventional dry process. Therefore, the UO₂ powder obtained by the process of the present invention is suitable for fabrication of nuclear fuels.

                  TABLE 1                                                          ______________________________________                                                                     com-                                                             present invention                                                                            parative                                                         case 1                                                                               case 2  case 3  case                                       ______________________________________                                         1st fluidized bed                                                              bed temperature (°C.)                                                                   280     280     280   280                                      UF.sub.6 rate (g/min)                                                                          90      90      90    90                                       atomizer steam rate                                                                            14      14      14    0                                        (g/min)                                                                        fluidizing gas velocity                                                                        25      25      25    25                                       (cm/s)                                                                         UO.sub.2 F.sub.2 production rate                                                               79      79      79    79                                       (g/min)                                                                        use of twin fluid atomizer                                                                     used    used    used  not used                                 2nd reactor                           not used                                 reaction temperature (°C.)                                                              20      20      20                                             UO.sub.2 F.sub.2 rate (g/min)                                                                  79      79      79                                             water rate (g/min)                                                                             9       9       9*                                             formed UO.sub.2 F.sub.2 hydrate                                                                86      86      86                                             (g/min)                                                                        3rd fluidized bed                     not used                                 bed temperature (°C.)                                                                   170     170     170                                            UO.sub.2 F.sub.2 hydrate (g/min)                                                               86      86      86                                             fluidizing gas velocity                                                                        20      20      20                                             (cm/s)                                                                         formed UO.sub.2 F.sub.2 anhydride                                                              79      79      79                                             (g/min)                                                                        4th fluidized bed                     not used                                 bed temperature (°C.)                                                                   not     500     500                                                            used                                                           UO.sub.2 F.sub.2 rate (g/min)                                                                          79      79                                             fluidizing gas velocity 20      20                                             (cm/s)                                                                         formed UO.sub.3 /U.sub.3 O.sub.8 (g/min)                                                               69/4    69/4  79                                       5th fluidized bed                                                              bed temperature (°C.)                                                                   600     600     560   600                                      UO.sub.3 /U.sub.3 O.sub.8 rate                                                                 76      73      79                                             fluidizing gas velocity                                                                        20      20      20    20                                       (cm/s)                                                                         hydrogen/steam (mol. ratio)                                                                    1/1     1/1     1/1   1/1                                      formed UO.sub.2 (g/min)                                                                        69      69      69    69                                       ______________________________________                                          *water added with ammonia 1%                                             

                  TABLE 2                                                          ______________________________________                                                        present invention                                                                case   case   case comparative                                Characteristics of UO.sub.2 powder                                                              1      2      3    case                                       ______________________________________                                         bulk density (g/cm.sup.3)                                                                       2.1    2.0    1.9  2.8                                        specific surface area (m.sup.2 /g)                                                              2.6    2.9    3.1  1.0                                        mean partice size (μm)                                                                       96     98          145                                        U content (% U)  87.9   87.8   87.7 87.8                                       residual fluorine contents                                                                      58     41     38   278.0                                      (ppm)                                                                          ______________________________________                                     

What is claimed, is:
 1. A process for conversion of UF₆ to UO₂ by using a fluidized bed reaction apparatus comprising(a) a first step of pyrohydrolizing gaseous UF₆ and steam in said fluidized bed to obtain UO₂ F₂ particles at temperatures of less than 400° C., (b) a second step of hydrating said UO₂ F₂ particles with water to UO₂ F₂ hydrate at temperatures of less than 100° C., (c) a third step of dehydrating said UO₂ F₂ hydrate by heating to UO₂ F₂ anhydride at temperatures of less than 200° C., (d) a fourth step of reducing said UO₂ F₂ anhydride with hydrogen gas or a mixture of hydrogen gas and steam to convert to UO₂ powders at temperatures of less than 700° C.
 2. A process for conversion of UF₆ to UO₂ by using a fluidized bed reaction apparatus comprising(a) a first step of pyrohydrolizing gaseous UF₆ and steam in said fluidized bed to obtain UO₂ F₂ particles at temperatures of less than 400° C., (b) a second step of hydrating said UO₂ F₂ particles with water to UO₂ F₂ hydrate at temperatures of less than 100° C., (c) a third step of dehydrating said UO₂ F₂ hydrate by heating to UO₂ F₂ anhydride at temperatures of less than 200° C., (d) a fourth step of calcining said UO₂ F₂ anhydride with steam to convert to UO₃ or a mixture of UO₃ and U₃ O₈ particles at temperatures of less than 700° C., (e) a fifth step of reducing said UO₃ or said mixture of UO₃ and U₃ O₈ with hydrogen gas or a mixture of hydrogen gas and steam to convert to UO₂ powders at temperatures of less than 700° C.
 3. A process for conversion of UF₆ to UO₂ by using a fluidized bed reaction apparatus comprising(a) a first step of pyrohydrolizing gaseous UF₆ and steam in said fluidized bed to obtain UO₂ F₂ particles at temperatures of less than 400° C., (b) a second step of hydrating said UO₂ F₂ particles with a solution added with ammonia, oxalic acid or hydrogen peroxide to UO₂ F₂ hydrate at temperatures of less than 100° C., (c) a third step of dehydrating said UO₂ F₂ hydrate by heating to UO₂ F₂ anhydride at temperatures of less than 200° C., (d) a fourth step of reducing said UO₂ F₂ anhydride with hydrogen gas or a mixture of hydrogen gas and steam to convert UO₂ powders at temperatures of less than 700° C.
 4. A process for conversion of UF₆ to UO₂ by using a fluidized bed reaction apparatus comprising(a) a first step of pyrohydrolizing gaseous UF₆ and steam in said fluidized bed to obtain UO₂ F₂ particles at temperatures of less than 400° C., (b) a second step of hydrating said UO₂ F₂ particles with a solution added with ammonia, oxalic acid or hydrogen peroxide to UO₂ F₂ hydrate at temperatures of less than 100° C., (c) a third step of dehydrating said UO₂ F₂ hydrate by heating to UO₂ F₂ anhydride at temperatures of less than 200° C., (d) a fourth step of calcining said UO₂ F₂ anhydride with steam to convert to UO₃ or a mixture of UO₃ and U₃ O₈ particles at temperatures of less than 700° C., (e) a fifth step of reducing said UO₃ or said mixture of UO₃ and U₃ O₈ with hydrogen gas or a mixture of hydrogen gas and steam to convert to UO₂ powders at temperatures of less than 700° C.
 5. A process for conversion of UF₆ to UO₂ powders as claimed in any one of claim 1-claim 4, wherein said gaseous UF₆ and steam are fed into said fluidized bed by using a twin fluid atomizer.
 6. A process for conversion of UF₆ to UO₂ as claimed in any one of claim 1-claim 4, wherein said hydration of said UO₂ F₂ particles to UO₂ F₂ hydrate is carried out with steam added. 